TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas

Page 16
It is difficult to measure the current through the selectors, due to the smallness
and the fact that each impulse is of short duration and in opposite direction to the
preceding one. However, with a centre zero 10 milliampere-scale ammeter the swing
of the needle on each side of the zero point should be equal for the regular stepping
impulses. The value of the reading will depend on the damping of the meter and a
minimum value only can be obtained by trial.
If a voltmeter is used to measure the potential at that station, a high-resistance
meter, of not less than 15,000 ohms, must be used. A centre zero scale meter is
preferable and, in this case also, the swing of the needle on each side of the zero point
should be equal for the regular stepping impulses. The amount of the swing will
somewhat depend on the damping of the meter and a definite value cannot be given.
A minimum value can be obtained by trial.
As a general rule, adjustment of the selector in the field is not recommended. If
the cause of failure of operation of the selector is quite obvious there is no objection
to correcting the trouble at once. Extreme care should be taken in adjusting the
selector. If the cause of the failure is not easily seen or corrected, we recommend that
a spare selector be substituted in its place and the faulty selector returned to Standard
Telephones and Cables Limited for investigation. When a selector is returned, a
statement of the conditions and, as nearly as possible, the manner in which the
selector acted, should be sent along with the selector to aid in determining the cause
of failure.